This invention relates to a method of minimizing the processing steps normally involved in the recovery of hydrogen chloride in a process of hydrolyzing chlorosilanes to produce a hydrolyzate containing polysiloxanes.
The manufacture of polydimethylsiloxane polymers is a multi-step process. The hydrolysis of chlorosilanes obtained from the direct process is well known in the art and yields a mixture of cyclic and linear silanol-stopped oligomers called hydrolyzate. In some instances, chloro-stopped polymers are also obtained.
The ratio of cyclic to linear oligomers, as well as the chain length of the linear siloxanes, is controlled by the conditions of the hydrolysis, such as the ratio of chlorosilane to water, temperature, contact time, and solvents. Commercially, the hydrolysis of dimethyldichlorosilane is performed by either a batch or a continuous process. In the typical industrial operation, dimethyldichlorosilane is mixed with water in a continuous reactor. The mixture of hydrolyzate and aqueous HCl is separated, preferably by using a simple decanter, which is essentially maintenance-free. However, other means may also be utilized, including variations which combine coalescence technology with gravity separation. Commercially available multistage coalescer separators have been configured using replaceable porous media, to first coalesce and separate the silicone phase from the bulk continuous aqueous phase, and further refine by separating the fine dispersion of aqueous non-continuous phase from the silicone using hydrophobic media. HCl gas containing trace amounts of water is removed, and can be converted to methyl chloride, which can then be reused in the direct process. The hydrolyzate is washed for removal of residual acid, optionally neutralized with base addition or ion exchange technology, dried, and filtered. The typical yield consists of about 35-50 percent of cyclic oligomers, and the remainder consists of linear oligomers. Typically, the cyclic oligomers and the linear oligomers are subsequently separated by distillation. Water can be added to the hydrolyzate, the cyclic oligomers, or the linear oligomers for additional chloride removal.
The complete conversion of dimethyldichlorosilane to only linear oligomers is also possible in the continuous hydrolysis operation. In this operation, the cyclic oligomers are separated from the linear oligomers by a stripping process, and the cyclic oligomers are mixed with the dimethyldichlorosilane. This mixture undergoes equilibration to chloro-terminated oligomers, and is subsequently hydrolyzed. The silanol-stopped linear oligomers are then used in the manufacture of other silicone polymers. Typically, these silanol-stopped linear oligomers are reacted with a suitable endblocking agent such as hexamethyldisiloxane in the presence of a catalyst to obtain low and medium viscosity trimethysiloxy terminated polydimethylsiloxanes.
There is a need in the art for a simplified process that integrates the multi-step process into fewer unit operations. This can be achieved according to this invention by running the process under certain prescribed conditions, feed rates, and inputs into certain equipment for carrying out the processing functions. The simplified process herein provides an economic benefit in reduced capital intensity.